1. Field of the Invention
The present invention relates to an intermittent operation communication device and a communication system therefor, and more particularly to an intermittent operation communication device applicable to a communication system such as a sensor network comprising a plurality of spatially-dispersed wireless communication nodes, each of which periodically repeats an intermittent operation including beacon transmission and reception, and sleep.
2. Description of the Background Art
In mutual data communication performed by a plurality of nodes wirelessly, intermittent operation communication procedures have been researched and developed in order to save electric power consumption of the nodes by periodically turning off a power supply for an unused node. One of the procedures is disclosed in En-Yi A. Lin, et al., “Receiver Initiated Rendezvous Schemes for Sensor Networks” In Proc. of IEEE Globecom 2005, St. Louis, Mo., USA, November 2005. In this procedure disclosed, a node performs periodical and intermittent reception, and transmits a beacon packet right before the reception so as to inform its adjacent nodes of the reception timing. Accordingly, the node performs beacon transmission and enters a reception wait mode periodically, and, in any other periods than those for the beacon transmission and the reception wait mode, its device relating to the communication is basically powered off, i.e. the node is in its sleep mode. With such a procedure, when generating traffic, transmission should be performed while a destination is not in its sleep mode.
FIG. 2 is a timing chart for use in understanding the operation of the nodes in Lin, et al., on and after traffic is generated. Each of these nodes basically transmits a beacon packet for informing its reception timing, and enters its reception wait and sleep modes in a period. The figure shows a case where a node A in its sleep mode has traffic for transmitting data to another node B, i.e. the node A is a transmission node and the node B is a destination node. In this case, at the time the traffic is generated, the node A turns on the power of its receiver and shifts to its reception wait mode to attempt receiving a beacon packet from the destination. As soon as the node A receives a beacon packet from the node B, the node A transmits a data packet to the node B, and the node B receives a data packet from the node A right after the transmission of the beacon. Each period where the node A or B is in the reception wait mode is shown in FIG. 2 as a reception period (R).
However, such a conventional intermittent operation communication procedure has a problem, as seen from FIG. 3, that two nodes attempt transmission to the same destination node in the same period, thereby causing collision on the communication. FIG. 3 shows a state that each of the nodes A and B has traffic, meant for another node C, generated in the same period, and transmits a data packet to the node C at the same time right after receiving a beacon from the node C, thereby causing collision on the data packets. The solution to this problem is also described in Lin, et al., which describes a procedure provides, instead of transmitting a data packet immediately after receiving a beacon packet, an interval for random time between transmission of a beacon packet and reception of a data packet and shifting transmission timing of a data packet from reception time of a beacon packet by a random number of time slots, i.e. a contention window to thereby prevent a collision.
However, employing the procedure for preventing the collision described in Lin, et al., has a problem that one cycle of periodical operation includes a beacon transmission period, a random reception period for time slots and a fixed reception period, thereby causing an operation cost to increase by addition of the random reception period, i.e. a random wait period, to decrease time available for the sleep mode and to reduce the advantage of electric power saving.
Therefore, an intermittent operation communication device and a communication system are expected that are capable of preventing data collision without reducing time for the sleep mode.